In recent years, secondary lithium batteries have been noted because of their advantages, i.e., high charge-discharge voltage and large charge-discharge capacity. Since the secondary lithium batteries have a high energy density that allows the reduction of the size and weight thereof, they have been used in personal computers and communication business apparatus such as a portable telephone and can be expected to be used as a power supply for electric automobiles, etc. in the near future.
As the positive electrode material for secondary lithium batteries there have been considered promising lithium manganate (LiMn.sub.2 O.sub.4) which is a lithium-manganese oxide having a spinel structure instead of lithium cobaltate (LiCoO.sub.2), which had heretofore been used.
Particularly noted among the Li-Mn-O system are those obtained by replacing the Mn site of LiMn.sub.2 O.sub.4 with an extremely small amount of Li ions as represented by the composition formula Li.sub.1+x Mn.sub.2-x O.sub.4 (in which x represents a number of from more than 0 to less than 0.03) because of improved cycle durability of charge/discharge (Y. Gao and J. R. Dahn, J. Electrochem. Soc., 143, 100, 1996). The improvement in cycle durability is attributable to the reduction of crystal lattice distortion accompanied with charge/discharge, i.e., elimination/insertion of Li ions.
On the other hand, JP-A-7-254403 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses compounds obtained by partially substituting fluorine (F) for oxygen in LiMn.sub.2 O.sub.4 compound. is These compounds are represented by the composition formula Li.sub.x Mn.sub.2 O.sub.4-a F.sub.b (in which x represents a number of from more than 0 to not more than 1.02; a represents a number of not more than 0.05; and b represents a number of from not less than 0.01 to less than 0.1). These compounds are intended to inhibit self-discharging during charging, making it possible to secure discharge capacity, particularly under high temperature conditions.
However, the former compound, i.e., Li.sub.1+x Mn.sub.2-x O.sub.4 as a spinel structure having a slight excess of Li has a serious disadvantage that it has a reduced discharge capacity. For the stoichiometric LiMn.sub.2 O.sub.4 spinel, the distribution of cations is represented by (Li) [Mn.sub.2 ]O.sub.4, where () shows the tetrahedral site or the A-site and [ ] the octahedral site or the B-site respectively. So, the distribution of the cations in a Li.sub.1+x Mn.sub.2-x O.sub.4 spinel can be represented by (Li) [Li.sub.x Mn.sub.2-x ]O.sub.4 ; that is, Mn ions at the B-site are partially substituted with Li ions. From now on, we will call the Li ions at the B-site as the excess Li ions; and the amount of the excess Li ions are given by x.
As x increases, the average valence of Mn ions in Li.sub.1+x Mn.sub.2-x O.sub.4 increases from 3.5 for LiMn.sub.2 O.sub.4 with the increase of x. The charge/discharge capacity for Li.sub.1+x Mn.sub.2-x O.sub.4 depends on the amount of Li ions, which are intercalated reversibly into Li.sub.1+x Mn.sub.2-x O.sub.4. In particular, the capacity at around 4V with respect to Li metal is generated by a redox reaction between Mn.sup.3+ and Mn.sup.4+. Therefore, the increase in the average valence of Mn ions leads to a decrease in the charge/discharge capacity of the battery.
The relationship between the amount of excess Li and the theoretical capacity (mAh/g) is shown in FIG. 1. Referring to Li.sub.1+x Mn.sub.2-x O.sub.4 sample (material c) in FIG. 1, it can be seen that as the amount x of excess Li increases, the theoretical capacity decreases linearly. When the amount x of excess Li is 0.03, the theoretical capacity of Li.sub.1+x Mn.sub.2-x O.sub.4 sample is about 92% of that of the sample when x is 0.
This capacity drop has heretofore been permitted for the improvement of durability. However, this capacity drop impairs the advantage-of secondary lithium battery, i.e., high energy density. This is a serious problem from the standpoint of application to power supply for portable electronic apparatus or electric automobile.
On the other hand, the latter compound, i.e., Li.sub.x Mn.sub.2 O.sub.4-a F.sub.b, which is obtained by partially substituting fluorine for oxygen has no Li substituted for the Mn site. Therefore, this material is not liable to drop of initial charge/discharge capacity. However, this material is disadvantageous in that the discharge capacity is drastically reduced in repeating a charge/discharge cycle. Furthermore, it is considered difficult in ordinary solid phase reaction process to substitute fluorine for oxygen in a spinel structure.